Active transport of organic anion OA) and cations (OC) from CSF to blood is a critical determinant of the concentration of potentially toxic neurotransmitter metabolites, drugs, and xenobiotics within the brain. We have have used a combination of isolated membrane vesicles, tissue fragments in vitro, and primary culture to study xenobiotic transport across the blood-CSF barrier (choroid plexus). Our working hypothesis was that the transport mechanisms utilized by the plexus would parallel those of the kidney, but with the polarity reversed since the direction of transport is reversed (i.e., from CSF to blood rather than from blood to urine). Primary focus has been on the mechanism and subcellular location of OA transport by the plexus. Using either p-aminohippurate (PAH), the standard renal substrate, and the anionic herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D) (which is much more effectively transported by the choroid plexus), we have been able to demonstrate that apical transport of OA is mediated by OA/alpha-ketoglutarate (alphaKG) exchange ? the same mechanism used at the basolateral face of the renal epithelium (JBC, 1999). Consistent with the functional data, the green fluorescent protein/renal OA/alphaKG fusion protein was expressed exclusively at the apical face of rat plexus tissue transfected in vitro with OA/alphaKG cDNA. Thus, choroid plexus does use an identical mechanism to that found in the kidney for elimination of foreign OA, but it is expressed in the opposite (i.e. apical) membrane (JBC, 1999). However, initial studies of the OC system in both isolated apical membrane vesicles from the bovine plexus or in primary cultures of choroid plexus cells from neonatal rats indicate the presence of proton/OC exchange at the apical membrane, i.e., the same transport activity seen in the apical membrane of the proximal tubule. This finding indicates that choroid plexus has the same polarity with respect to OC transport as the renal tubule. Since the direction of transport is reversed (CSF to blood), OC transport must take place by a mechanism distinct from that used in the kidney. Other results in the cultured cells suggest that vesicular trafficking may participate directly in net transepithelial secretion and that nocodazol, a microtubule disrupting agent known to block trafficking in other systems, blocks basolateral release of vesicular OC (AJP, 1999) . We have also begun to assess OC transport in cultured human retinal pigmented epithelium (RPE) from the eye using verapamil as substrate (JPET, submitted). As judged by its unique substrate specificity, RPE verapamil transport appears to be a novel system. Nevertheless, like CP TEA transport, this apical OC transport process appears to be driven by proton exchange.